1. Field of the Invention
The field of the invention is spectral imaging, specifically spectral measurement which utilizes optical interference to determine spectral content, and which is capable of determining the spectral content at every point in a one- or two-dimensional image or scene.
2. Description of the Related Art
Optical interference is widely used in instruments such as the Michelson interferometer, the Mach-Zender interferometer, the Twyman-Greene interferometer, the Sagnac interferometer, and others. These divide incident light into two or more beams traveling along different paths, which are then recombined. An optical path difference is developed between the paths, which results in constructive or destructive interference, depending on the wavelength of light and on the optical path difference (OPD). The intensity pattern resulting from this interference is termed an interferogram. By observing the interferogram while varying the path difference, one can deduce the wavelength of light present in a monochromatic incident beam, or the amount of each wavelength component in a polychromatic beam. This spectrum is obtained by means of a Fourier transform of the intensity signal, for which reason these instruments are often termed Fourier transform spectrometers, or FTS instruments.
The spectral resolving power of an FTS instrument depends directly on the range of OPD that can be produced. Usually, the OPD is varied by mechanical means, such as actuators or piezeo-electric crystals. This involves the motion or rotation of one or more optical elements, such as mirrors or windows, which leads to high cost and complexity given the need to control the OPD variation to a small fraction of a wavelength of light. Other means are also used for varying path length. In some cases an electro-optic material is present in one or more of the optical paths, and the OPD is varied electro-optically. However, this normally results in a limited range of adjustment, as the modulation range of most electro-optic modulators is limited to approximately one wavelength of light. In other cases, one path contains a cell or vessel which may be evacuated or pressurized with gases, to produce an OPD change via the change in refractive index.
It is common practice to incorporate some means for measuring the change in OPD, often via a laser reference beam. In these arrangements, the laser travels along the same paths as the light being analyzed, or travels an equivalent path located adjacent the aperture region, and its output is observed as the OPD is varied. This is especially common in mechanically-varied systems, or in systems which provide a large change in OPD, for which it is otherwise difficult to determine the exact OPD achieved by the system. These systems are generally complex, mechanically delicate, and thermally sensitive.
Most FTS systems provide a spectrum for the incident beam as a whole, and cannot provide spectra for each point in a two-dimensional input scene; thus they are termed non-imaging spectrometers. However, a few imaging FTS systems have been built. Such systems typically sense the interferogram using a pixelated detector such as a CCD or CID sensor. Cabib teaches in U.S. Pat. No. 5,835,214 the use of low-finesse Fabry-Perot interferometers and interferometers which split the incident beam into a finite number of beams, for the purpose of imaging an entire scene at once and obtaining spectral information about each pixel. Cabib describes several interferometers, in U.S. Pat. No. 5,835,214, in U.S. Pat. No. 5,784,162, and in U.S. Pat. No. 5,539,517. Depending on the optical design of the instrument, a given pixel on the sensor may correspond with a given point in the scene being imaged for all OPD settings, or it may not. For example, an instrument is commercially available from Applied Spectral Imaging, in Migdal Haemek, Israel. In this instrument, the relationship between pixel location and scene location varies as the OPD is changed. A careful accounting must be made to determine which pixel corresponds to which image point, for each different OPD, before the spectra can be calculated. A related problem arises because the sensor has discrete pixels: as the OPD is varied, the sensor location corresponding to a given scene location will move from being within a given pixel, to the adjacent region which lies between pixels. This causes image smear, and mixing of the spectral content of adjacent pixels. Further, it is not easy to acquire images of objects that move between successive exposures, since registration error cannot be corrected by a simple Cartesian shift.
Another type of interferometer described by Buican in U.S. Pat. No. 4,905,169 avoids many of the problems of classical interferometers. It uses a photo-elastic modulator (PEM) or equivalent device to imprint a time-varying retardance on a beam of polarized incident light, which then passes through a linear polarizer after which its intensity is measured at a photodetector. By Fourier analysis of the intensity signal, the spectral content of the incident light is determined. This instrument acts as an interferometer, based on polarization. The time-varying retardance is equivalent to an OPD between the components polarized along the ordinary and extraordinary axes of the PEM, and the analysis at the polarizer generates the equivalent of an interferogram.
Buican""s instrument offers the benefits of simplicity, absence of moving parts, and ruggedness. Consequently, it can be built more economically than present-day alternatives. However, there are a few severe limitations. PEM devices provide an adequate range of OPD only when operated in resonant mode. This means that the glass or crystal element involved is excited with transducers at or near the frequency of mechanical resonance, which is typically in excess of 10 kHz, and more commonly in the range 50-80 kHz. Since the PEM undergoes an OPD excursion of up to 16 wavelengths, one would need to measure the interferogram intensity a minimum of 32 times per OPD sweep to achieve the Nyquist sampling criterion. Because the OPD is swept twice during each PEM oscillation, a total of 64 readings must be taken in 100 microseconds or less, or 1.6 microseconds per reading. This is possible with high-speed non-imaging detectors such as photodiodes or photomultiplier tubes (PMT), but not with imaging detectors like CCD or CID sensors. These sensors have much slower readout rates, since each readout involves digitizing many pixels. While very short exposure times are possible, the overall time per frame is normally 1000 microseconds or more when acquiring a continuous stream of images. This is 600xc3x97 too slow to use in Buican""s instrument.
Accordingly, Buican teaches that this instrument may be used in flow cytometry experiments as a non-imaging spectrometer. He further teaches in U.S. Pat. No. 5,117,466 that it is possible to combine this non-imaging instrument with optical scanning means, to obtain a two-dimensional image of a scene with spectra at each point. However, the need for scanning means subvert the inherent simplicity and ruggedness that this system offers.
Other spectral imaging systems have utilized band-sequential approaches, where an imaging detector is coupled with a spectral filter. Examples based on acousto-optic tunable filter (AOTF) elements include Lewis et. al., in U.S. Pat. No. 5,377,003, and Chao et. al. in U.S. Pat. No. 5,216,484.
Kaye teaches in U.S. Pat. No. 4,272,195 a wavelength measuring system comprising a single liquid crystal cell which is driven with a varying voltage while the incident light passes through polarizers on either side of the cell, through the cell, and onto a detector. By adjusting the drive voltage, the retardance of the cell is altered, producing a series of maxima and minima at the detector. The maxima and minima are counted to determine the wavelength of quasi-monochromatic incident light. For light with a finite bandwidth, an estimate of the bandwidth is obtained by observing the decrease in contrast at higher retardances relative to low retardance. The system is not capable of spectral analysis, nor of identifying plural wavelengths in an optical beam, as the only information available is the number of maxima and minima during an overall retardance excursion; hence only the mean-center wavelength is sensed.
Kaye teaches in U.S. Pat. No. 4,444,469 a fluorescence imaging system where a liquid crystal variable retarder is used with polarizers to selectively block a given excitation wavelength, while transmitting nearby wavelengths at which there is fluorescence emission. The retardance is stepped by integral multiples of the excitation wavelength, so that the excitation band remains blocked in all cases, while the system exhibits a variety of spectral transmission responses in the emission band. This overcomes the limitation that, for a given retardance setting, certain emission wavelengths are also extinguished, and it is said to provide an average transmission of approximately one-half for most wavelengths in the emission light. No attempt is made to utilize the variation in detected signal versus retarder setting in an effort to determine the spectral content of the fluorescent light. Kaye also teaches that a fixed retarder element such as a quartz waveplate or the like may be used in series with the liquid crystal cell. This causes the system to operate in higher order, and the spacing between transmitted and extinguished wavelengths is reduced. This enables use with fluorescent probes having a short Stokes shift.
Mercer teaches in U.S. Pat. No. 5,689,314 a point-diffraction interferometer which uses liquid crystal material to preferentially introduce a phase shift in the region surrounding a glass or plastic sphere embedded in the liquid crystal material. The sphere acts as a pinhole in this non-imaging system. Polarized light is divided into a reference beam, which passes through the glass sphere, and an object beam which passes through the liquid crystal material and which may be phase shifted by the electro-optic action of that material. The polarization axis of the incident light is parallel to the slow axis of the liquid crystal material, so a pure phase shift is produced, rather than a change in polarization state.
Satorius et. al. teaches in U.S. Pat. No. 5,838,441 an optical coherence detector which uses a fixed retarder and a variable retarder to detect coherent light in a two dimensional scene, with very wide field-of-view. The fixed retarder introduces a path delay that exceeds the coherence length of non-coherent light, but is shorter than the coherence length of the coherent light being sought. The variable retarder is cycled to adjust the overall retardance, and the resulting beam is checked for modulation, which would indicate the presence of coherent light.
Bendall teaches in U.S. Pat. No. 5,600,440 a Michelson interferometer with liquid crystal phase modulators in each arm of the interferometer. Pixilated liquid crystal elements provide a one- or two-dimensional linear array of pixels each of whose phase may be independently adjusted. The spectral resolution is shown as related to the number of distinct pixel regions in the liquid crystal cells, but the reason for this is unclear. A single-pass version uses only a single beam; an interferogram is developed by unspecified means. One embodiment of the single-beam, single-pass instrument uses a pair of adjacent sequential liquid crystal cells, driven identically, with orthogonally-oriented slow axes, to produce pure phase modulation. The reliance on a Michelson design renders the instrument thermally-sensitive, and requires fabrication of many precision optical surfaces; these concerns are in no way ameliorated through the use of liquid crystal elements.
Others, including Kaye in U.S. Pat. No. 4,394,069 and Tatsuo Uchida in Laser Focus World, May 1999, page 65, have taught the use of N liquid crystal cells and N+1 polarizers in optical series to form a Lyot filter. The thickness of the liquid crystal layers is carefully maintained in a ratio of integers or half-integers. When like voltages are applied to all cells, they express retardances which are in the same integral or half-integral ratios. By proper design using these cells sandwiched between polarizers, a Lyot filter is formed which can be easily tuned by varying the voltage that is applied to the ensemble of cells. Kaye teaches the use of this assembly as a tunable spectrometer. However, construction of an instrument with high resolution is difficult, due to mismatch between cells, thermal drift, and component tolerances. The same factors limit the wavelength accuracy of a spectrometer built on these principles.
Itoh et. al. teach in Optics Letters Volume 15(11), pp 652-654 (1990) the use of a nematic liquid crystal cell in reflection mode to act as a Fourier-transform spectrometer in an imaging system using a CCD detector. But they point out that their system suffers from strong dependence on incident angle, leading to a severely limited field-of-view, which is asymmetric and not easy to study or remove by post-processing of the interferograms. Further, the system is prone to drift and does not produce a stable OPD as the ambient temperature is varied.
Sharp et. al. teach another birefringent interferometer in SPIE Proc. 3384, Conference on Photonic Processing Technologies and Applications II, pp. 161-171 (1998). This device incorporates quartz plates or other fixed retarders which are switched using achromatic half-wave plates based on ferroelectric liquid crystal cells. While it has rapid time response and no field-of-view limitations are described, only discrete values of OPD are produced by this system, corresponding to the sum and difference values of the retarder elements involved. The discrete OPD of this system is a severe limitation since it is a benefit to be able to produce arbitrary values of OPD, or at least OPD values that are precisely even in their spacing. Achieving precisely even OPD spacing with this system would require fabrication of the fixed retarders to exacting tolerances, which is costly. The apparatus further incorporates an achromatic quarter-wave plate to digitally switch the retardation, wavelength-independently, in quarter-wave steps. Yet this does not ameliorate the problems brought on by discrete OPD steps. Further, since it produces a fixed polarization rotation (on the Poincare sphere), this element effects an OPD that is proportional to wavelength. Since it does not provide the same OPD for all wavelengths (even in the absence of dispersion), but rather provides a constant degree of polarization rotation independent of wavelength, it is not clear whether any meaningful spectral information is gained from the use of this element.
Miller teaches in U.S. Pat. No. 5,247,378 the construction of Lyot and Solc filters using liquid crystal cells in combination with fixed retarders. This provides for retardances that can be large, yet variable, from which a liquid crystal tunable filter (LCTF) of high spectral resolution can be built. The retardance of each liquid crystal cell is sensed by an electrical scheme, enabling precise tuning despite thermal drift, aging, and the like. In distinction to Kaye U.S. Pat. No. 4,394,069, different voltages are required at each stage to achieve proper tuning. The drive levels are calculated by a microprocessor, based on the properties of the fixed retarders and liquid crystal cells involved. Kaye teaches in U.S. Pat. No. 4,497,542 that two nematic liquid crystal cells of the type conventionally termed xe2x80x98flat-fieldxe2x80x99 cells, constructed with mirror-symmetric pre-tilt angles, may be placed in series to provide an overall retardance that is significantly freer of off-axis variation in retardance than a single flat-field cell. This benefit is especially great under partial-drive conditions.
Miller teaches in U.S. Pat. No. 4,848,877 the use of a monochromatic light beam to determine the retardance of a nematic variable retardance liquid crystal cell while it is in use, or at intervals. The beam is polarized prior to entering the liquid crystal cell, preferably at or near an axis of 45 degrees with respect to the liquid crystal director axis. It is then analyzed and its intensity is detected, from which the transmission and cell retardance are determined.
So, while interferometers, PEM-based polarization interferometers, AOTFs, LCTFs and various liquid crystal arrangements have been taught by the prior art, all have severe limitations. Those which provide for spectral imaging by interference means such as Michelson, Sagnac, or other interferometers suffer from high cost and complexity. PEM systems are not compatible with imaging detectors, and require a scanning mechanism if an image is sought. AOTFs and LCTFs have low throughput and, since they reject all light that lies outside of their passband, fail to make use of all the information in the incident beam. Coherence detectors and wavelength detectors do not provide spectral data for non-monochromatic signals. Prior-art liquid crystal interferometers do not provide a useful field-of-view, or are not thermally rugged, or do not provide continuously variable OPD. The fluorescence system of Kaye provides only for collection of fluorescence emission in the face of an excitation signal, without any means for determining the spectrum of emission light. None provides a rugged, economical, precise means for spectral imaging.
It is the aim of the present Invention to provide a simple, low-cost means for spectral imaging. It is a further aim to provide for enhanced ruggedness over mechanically-operated FTS instruments. It is yet another aim to provide this capability in a device which is smaller than any prior-art device, and preferably at least an order of magnitude smaller than any prior art device, so that it can be incorporated into microscopes, telescopes, and the like, without difficulty.
The invention consists of an apparatus and method for providing spectral information via one or more liquid crystal variable retarder cells situated between a pair of polarizers, to form a polarization interferometer that is continuously variable over a substantial range of OPD. This interferometer is used together with a detector to read overall intensity while the liquid crystal retardance is varied by known amounts. From the pattern of variation with OPD, the spectrum is obtained using Fourier transform methods.
The assembly forms an imaging spectrometer that is simple, economical, and capable of precise spectroscopic analysis. It provides nearly diffraction-limited image quality, and can be built using two-dimensional pixelated detectors such as CCD and CID sensors. This arrangement yields spectra for every point in a complex scene, using very simple equipment and conventional imaging detectors.
It is possible to incorporate fixed retarders such as quartz or calcite waveplates into the polarization interferometer, to achieve a greater range of OPD adjustment and realize higher spectral resolution. Several cells may be combined in series without significantly degrading the image quality, OPD selection, or off-axis properties. The invention preferably includes a reference beam, such as from a laser diode, to provide in-situ monitoring of the OPD.
Unlike the Sagnac interferometer of Cabib, a given sensor pixel corresponds to a single scene location, regardless of the OPD setting of the liquid crystal elements. As there are no moving parts, nor any electro-optic deflection, there is no shift in the image as the OPD is varied. Image quality is essentially diffraction-limited. Calibration of wavelength scale is straightforward and reproducible, and there is essentially no thermal drift. Similarly, the calibration of intensity (or transmission) values is direct.
At the liquid crystal cell setting corresponding to an OPD of zero, essentially all light is transmitted to the sensor, independent of wavelength. Thus, for focusing and other operations, one may image all points in the scene directly without any attenuation.
In practicing the present invention, the polarization interferometer can be placed directly adjacent the detector, for analyzing the spectral quality of luminous or fluorescent objects. Alternatively, it may be placed adjacent a lamp and used to filter the light which illuminates a scene; the scene may then be viewed with a detector using essentially conventional optics, and spectra determined for each point in the scene.
Efficiency of the present invention is approximately 80 percent, broadband. This exceeds the theoretical limit of 50% for interferometers such as Michelson or Sagnac types. Further, the instant invention, although based on polarization interference, can be constructed to utilize both polarization states of light in an incident beam. Thus, it can achieve throughputs of 80% even for unpolarized scenes. It can therefore be used in low-light settings such as fluorescence without a penalty in efficiency. It can also be used in remote sensing, medical imaging, bright-field microscopy, colorimetry, general spectroscopy, and other applications.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.